The present invention relates to the production of synthesis gas from natural gas by partial oxidation. Partial oxidation is a widely used process which yields synthesis gas having a hydrogen to carbon monoxide ratio near 2, which is a particularly suitable synthesis gas for the production of methanol, dimethyl ether, heavier hydrocarbons by the Fischer-Tropsch process, and other chemical products. The partial oxidation process uses oxygen provided by an air separation system to convert a wide variety of feedstocks ranging from methane to heavier hydrocarbons into synthesis gas. The efficient operation of the air separation system and integration of the system with the partial oxidation process are important factors in the overall cost of producing synthesis gas.
Natural gas typically contains components which boil above the boiling point of methane such as water, C.sub.2.sup.+ hydrocarbons, carbon dioxide, and sulfur-containing compounds. Natural gas also may contain components such as nitrogen and helium which have lower boiling points than methane. The operation of partial oxidation processes using natural gas feed is affected minimally by the presence of components heavier than methane in the feed, so feed pretreatment often is not needed. In some cases it may be desirable to remove sulfur-containing compounds from the feed gas prior to partial oxidation, for example when catalytic partial oxidation is used.
Components in the natural gas feed which are lighter than methane and which act essentially as inert diluents, usually nitrogen and occasionally helium, are undesirable for a number of reasons. These diluents reduce the effective partial pressure of methane in the partial oxidation reactor, increase the volume of feed and product gas to be handled, and dilute the synthesis gas used in downstream processes. Nitrogen may be undesireable in downstream processes for other reasons as well. Thus it will be preferred in certain cases to remove the diluent components from the natural gas feed prior to the partial oxidation reactor system.
Methods for removing nitrogen from natural gas, typically termed nitrogen rejection, are well known in the art as taught by the review article entitled "Upgrading Natural Gas" by H. Vines in Chemical Engineering Progress, November 1986, pp. 46-50. Other representative nitrogen rejection processes are disclosed for example in U.S. Pat. Nos. 4,411,677; 4,504,295; 4,732,598; and 5,617,741.
The air separation plant which provides the oxygen for the partial oxidation reactor also produces a nitrogen byproduct, and it is desirable to utilize this nitrogen byproduct when possible to reduce the overall cost of the synthesis gas and the products generated from the synthesis gas.
U.S. Pat. No. 5,635,541 discloses the use of an elevated pressure air separation plant to supply oxygen for natural gas conversion to higher molecular weight hydrocarbons. Elevated pressure nitrogen byproduct gas is utilized in several ways to improve the efficiency of the overall process. In one embodiment, the byproduct nitrogen is cooled by work expansion and contacted with water to produce chilled water used for cooling the air separation unit compressor inlet air. In another embodiment, the byproduct nitrogen is expanded to generate work to produce electricity or for gas compression. In an alternative mode, the nitrogen is heated by waste heat from the natural gas conversion process prior to expansion. U.S. Pat. No. 5,146,756 discloses an elevated pressure air separation system wherein byproduct nitrogen from the cold end of the main heat exchanger is work expanded and reintroduced into the exchanger to provide additional cooling for increased efficiency. Expanded and warmed nitrogen from this step can be used further for cooling at ambient temperatures to replace or reduce the use of cooling water. Alternatively, some of the pressurized ambient temperature nitrogen can be work expanded and further cooled for other uses outside of the air separation system.
It is desirable to reduce the capital and operating cost of a process plant for the partial oxidation of a natural gas feed to synthesis gas by integrating the operation of the air separation unit with the partial oxidation process and optionally with the synthesis gas consuming process. This can be achieved in part by efficient utilization of the nitrogen byproduct from the air separation system, particularly when this system generates a nitrogen byproduct at above atmospheric pressure. When the natural gas feed contains significant amounts of lower boiling components such as nitrogen, it is often desirable to pretreat the feed to remove this nitrogen, thereby reducing downstream equipment size and gas handling requirements. The invention described below and defined by the claims which follow offers an efficient method of integrating the air separation unit with the partial oxidation process by removing nitrogen from the natural gas feed utilizing byproduct nitrogen from the air separation unit.